The recent increase in small electrically-powered devices has increased the demand for very small electrochemical cells, usually disc-like or pellet-like in appearance, commonly referred to as a coin cell. Such cells, which are approximately the size of garment buttons, have diameters ranging up to about 1.0 inch and heights ranging up to about 0.60 inches. Because of their minute size, the power generating electrochemical reactions in button cells must be efficient and complete. Additionally, button cells must be manufactured with substantial precision to meet the geometric tolerances of the electrical device to avoid leakage of corrosive electrolytes.
For many such applications, the electrochemical cells of choice is lithium carbon monoflouride (Li/CF.sub.x) coin cells. These electrochemical cells are comprised of lithium anodes, carbon monofluoride cathodes and a electrolyte, usually a lithium salt. However, the use of LiCF.sub.x cells is limited by storage and use temperatures, i.e. cell performance deteriorates when the cells are stored at elevated temperatures above 60.degree. C. or used for extended periods at temperatures of 100.degree. C. or more. The deterioration of cell performance is unrelated to the fundamental electrochemical couple, which is capable of up to 181.degree. C. for the lithium and 450.degree.-650.degree. C. for the carbon monofluoride. Moreover, in characterizing carbon monofluoride using a differential thermal analysis, it was determined that the lithium fluoride (LiF) discharge product is stable in the carbon monofluoride at cathode temperatures up to 850.degree. C.
Typically lithium carbon monofluoride electrochemical cells are stored at temperature from -20.degree. C. to +60.degree. C. When stored at higher temperatures cell performance can be affected due to problems with three cell components--the seal, the separator, and the electrolyte. The seals of Li/CF.sub.x electrochemical coin cells are comprised of a polyolefin, usually polyropylene, grommet and a sealant. Any alteration of the grommet after the electrochemical cell has been manufactured will have a detrimental effect on the electrical and physical characteristics of the cell. Under conditions of high thermal stress, e.g., 125.degree. C., polypropylene has a tendency to flow and/or oxidize. Further, the oxidation reaction can be accelerated by the solvents of the electrolyte. When polypropylene grommets begin to flow, the compression crimp seals reduced, which in turn results in the electrical degeneration of the cell. In addition to sealing the electrochemical cell the polypropylene grommet controls the egress and ingress of water vapor to the cell as a function of the ambient humidity. In LiCF.sub.x electrochemical cells, the sealant is applied to the grommet to fill voids and irregularities between the grommet and the metal can and top of the cell. The prevalent sealant today is a bituminous solution, which has a low degree of thermal stability and tends to flow when subjected to extended periods of elevated temperatures. Such flow of the bitumen sealant can destroy the seal integrity, resulting in a change in the concentration of the electrolyte, which can result in a capacity degeneration.
The separator in LiCF.sub.x cells, typically made of non-woven polyproylene, acts as a mechanical cushion as well as an insulator between the anode and cathode. The separator also serves as the electrolyte absorbent necessary to maintain the desired ionic conductivity in the cell. Under conditions of extended high temperature storage, e.g., 125.degree. C. or higher, standard non-woven polypropylene tends to shrink. And because the polypropylene begins to melt at 150.degree. C., separators in LiCF.sub.x cells can melt when such cells are stored or used at high temperatures. Of course, melt down of the separator results in the irreversal degradation of the cell.
The electrolyte of a LiCF.sub.x cell is the cell's ionic conductor. Generally, in the lithium/carbon monofluoride cell chemistry, a lithium tetrafluoroborate (LiBF.sub.4) salt in 50/50 volumetric solution of propylene carbonate (PC) and dimethoxyetethane (DME) is used as the electrolyte. DME is highly volatile and, during use or storage at high temperatures of 125.degree. C. or more, can diffuse around and through the seal, which causes an increase in cell impedance and electrical degradation. At temperatures of 200.degree. C. or more, the propylene carbonate will decompose and/or an interact with the lithium tetrafluoroborate salt, causing the electrical degradation of the cell.
Accordingly, it is an object of the present invention to provide for a lithium carbon monoflouride electrochemical cell which may be stored and/or used for extended periods at high temperatures.
Still another object of the present invention is to provide for a high temperature lithium solid cathode cell which can be constructed according to known manufacturing techniques.
The above objects of the present invention will be more fully understood, and further objects and advantages will become apparent, from the following description of the invention.